Question 5 of 7: Section 5: Siliciclastic Reservoirs

Section 5: Siliciclastic Reservoirs (20 Marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Q5-1 — Definition of a delta. A delta is a discrete shoreline protuberance built where a river enters a standing body of water (sea or lake) and deposits sediment faster than wave, tidal and current processes in the basin can redistribute it, progressively prograding a wedge of fluvially-derived sediment out into the basin.

Q5-2 — Vertical log and GR profile, 25 m wave-dominated delta.

A wave-dominated delta progrades as a smoothly coarsening-upward succession because wave energy continuously reworks and winnows the delta front, building a well-sorted, sheet-like sand body: dark, bioturbated prodelta mud (below wave base) passes up into interbedded, wave-rippled silt/very-fine sand of the distal delta front, then into cleaner, hummocky-cross-stratified proximal delta front sand, capped by delta-plain/distributary-mouth-bar sand at the top. The gamma-ray log mirrors this directly: high GR (shale-dominated) at the base decreases upward as sand content increases, producing the classic funnel (upward-decreasing GR) log motif diagnostic of a progradational, coarsening-upward parasequence.

Q5-3 — Maximum and minimum porosity, spherical grain packing.

Given. Uniform, well-sorted spherical grains, repacked with no cementation or grain-size mixing.

Find. The maximum porosity (loosest stable packing) and minimum porosity (tightest packing) achievable purely by repacking.

Approach. Porosity is the void fraction of the packing's unit cell, $\phi = 1 - (\text{solid volume fraction})$; the two limiting packings are simple cubic (loosest) and rhombohedral (tightest, each sphere touching 12 neighbours).

1. Maximum porosity — simple cubic packing. A unit cube of side $2r$ contains exactly one sphere of radius $r$ (volume $\tfrac{4}{3}\pi r^3$): $$\phi_{max} = 1-\frac{\tfrac{4}{3}\pi r^3}{(2r)^3} = 1-\frac{\pi}{6}$$ $$\boxed{\phi_{max} = 1-\frac{\pi}{6} = 47.6\%}$$
2. Minimum porosity — rhombohedral (closest) packing. The rhombohedral unit cell has solid fraction $\pi/(3\sqrt{2})$: $$\phi_{min} = 1-\frac{\pi}{3\sqrt{2}}$$ $$\boxed{\phi_{min} = 1-\frac{\pi}{3\sqrt{2}} = 25.9\%}$$

Note that neither limit depends on grain size — only on packing geometry, which is why Q5-4 below attributes porosity control mainly to sorting/packing rather than to absolute grain size.

Q5-4 — Effect of grain size and sorting on porosity and permeability. For a well-sorted, uniform packing, porosity is essentially independent of absolute grain size — a packing of large spheres and a geometrically identical packing of small spheres have the same void fraction (Q5-3), because porosity is a packing-geometry property, not a size property. Permeability, however, depends strongly on grain size, because it scales with the square of the pore-throat radius (a Kozeny-Carman-type relationship): coarser, well-sorted sand has proportionally larger pore throats and therefore much higher permeability than finer sand of the same porosity. A change in sorting from well sorted to moderately sorted decreases both porosity and permeability: the wider grain-size distribution allows smaller grains to occupy the pore space between larger grains, directly reducing pore volume (porosity) and constricting/occluding the connecting pore throats (permeability) far more severely than it reduces porosity.

Q5-5 — Coal bed methane as an unconventional resource. Coal bed methane (CBM) is unconventional because the gas is stored predominantly by adsorption onto the internal surface area of the coal's organic matrix (micropores), not as free gas in conventional interparticle porosity, and the coal seam itself is simultaneously the source rock, reservoir and (self-sealing) trap — there is no discrete structural or stratigraphic trap and no separate top seal in the conventional sense. Production requires reducing reservoir (often initially water-saturated) pressure through dewatering to desorb the gas, rather than simply relying on a buoyant/pressure-driven flow from a conventional closure, and typically needs stimulation (hydraulic fracturing/cleat-network enhancement) because coal's natural permeability is low and highly stress-sensitive — all hallmarks of a continuous, unconventional accumulation rather than a discrete conventional pool.